1. Field of Application
The present invention relates to an electromagnetic switch to be connected in an electrical circuit, controllable for opening/closing switch contacts to interrupt/enable supplying of current by the electrical circuit to a load such as a DC motor.
2. Background Technology
An example of an electromagnetic switch is described in U.S. patent application publication No. 2009/0183595, referred to in the following as reference 1, with the switch being incorporated in a starter apparatus for the drive engine of a vehicle (where “vehicle” as used herein signifies an automotive vehicle, with “engine” signifying an internal combustion engine and “motor” signifying a DC electric motor). In that apparatus, a first solenoid actuates a pinion of a starter motor to become pressed against a ring gear of the vehicle engine. A second solenoid (of the electromagnetic switch) serves to open/close switch contacts, connected in a circuit which supplies current to the starter motor. The first solenoid and the second solenoid are controlled respectively independently. This enables the timings at which the pinion is actuated by the first solenoid and the timings at which current is supplied the starter motor by the action of the second solenoid to be respectively independently controlled. These timings can thus be optimally determined for the purposes of an idling stop system.
The function of an idling stop system installed in a vehicle are essentially as follows. When the vehicle becomes halted temporarily (e.g., at traffic lights or due to traffic congestion), the idling stop system automatically halts the supplying of fuel to the vehicle engine, stopping the engine. Thereafter when the vehicle driver performs some predetermined action which indicates that the vehicle is to be set in motion (e.g., releases the brake pedal, or shifts the automatic transmission to the drive range), the idling stop system automatically operates the starter apparatus to restart the engine.
Exhaust gas emissions can thereby be reduced and fuel consumption decreased, so that such idling stop systems have come into increasing use.
However, by comparison with a vehicle which does not incorporate such a system, an idling stop system has the disadvantage that the frequency of stopping/restarting the engine is increased considerably. Thus the frequency of using the starter apparatus is increased accordingly. When the starter apparatus of reference 1 is used with such an idling stop system, the frequency of opening/closing the switch contacts is increased by approximately a factor of 10, by comparison with a conventional system. Hence, the rate of wear of the switch contacts is increased accordingly, thereby substantially reducing the operating lifetime of the switch contacts.
This point will be described more specifically referring to FIG. 10, which is a cross-sectional view of the interior of a known type of electromagnetic switch. The configuration shown is basically identical to that of the electromagnetic switch 6 shown in FIGS. 1 and 2 of reference 1. In the electromagnetic switch 100 of FIG. 10, when a current is passed through a coil 110, a stationary iron core 120 becomes magnetized thereby pulling a plunger 130 along an axial direction. A pair of terminal bolts 150 and 160 are fixed in a plastic cover 140 (where “plastic” as used herein signifies polymer resin), and are respectively connected to stationary contacts 170 and 171.
The terminal bolts 150 and 160 consist of the B-terminal bolt 150, which is connected to the positive potential of the vehicle battery, and the M-terminal bolt 160 which is connected to the starter motor, i.e., is connected via an armature winding of the starter motor to the negative potential of the battery. The stationary contacts 170 and 171 are located within a contact chamber in the interior of the plastic cover 140, respectively attached (electrically connected) to the B-terminal bolt 150 and to the M-terminal bolt 160.
A movable contact 180 is located at the axially opposite side of the stationary contacts 170 and 171 from the plunger 130, and bears against an end face of the rod 190, which is fixedly attached at its opposite end to the plunger 130.
When current does not flow through the coil 110, the plunger 130 is urged axially rightward (as viewed in FIG. 10) by a return spring 210 which is located between the stationary iron core 120 and the plunger 130. In that condition, the movable contact 180 is held separated from the stationary contacts 170 and 171, so that the switch contacts are open. The terms “axial” and “axially”, as used herein in describing internal components of an electromagnetic switch, are to be understood as referring a direction parallel to a central axis of the plunger (i.e., parallel to the displacement direction of the plunger) of the electromagnetic switch.
When current is passed through the coil 110 thereby magnetizing the stationary iron core 120, the plunger 130 becomes attracted towards the stationary iron core 120 and so displaces the rod 190 axially leftward, compressing the return spring 210. A contact press spring 200 is thereby enabled to urge the movable contact 180 into electrical contact with each of the stationary contacts 170 and 171, so that the switch contacts become closed.
Over a period of use in which a large number of on/off switching operations have been executed, one or both of the stationary contacts 170 and 171 may become completely worn. Here, the term “completely worn” as applied herein to a stationary contact signifies that a part of the stationary contact has become worn in an axial direction by an amount equal to its (original) thickness. In practice, the stationary contacts 170 and 171 do not become worn at identical rates, with the rate of wear of the positive-side terminal being greater than that of the negative-side terminal. This is illustrated in FIG. 11, in which the first stationary contact 170, attached to the B-terminal bolt 150, has become completely worn, whereas the second stationary contact 171 remains only partially worn. For similar reasons (also as illustrated) the face region of the movable contact 180 which comes into direct contact with the second stationary contact 171 will become worn at a greater rate than the face region which contacts the first stationary contact 170.
When the switch contacts are closed, with part of the first stationary contact 170 in a completely worn condition such as is shown in FIG. 11, an outer side portion of the movable contact 180 (e.g., an upper side portion, as viewed in FIG. 11) may penetrate beyond the thickness of the first stationary contact 170, and thus may become tilted. In that condition, when the current flow through the coil 110 is then interrupted, an outer side portion of the movable contact 180 may become caught against the worn portion of the first stationary contact 170. When this occurs, in the worst case, the restoring forces applied by the return spring 210 may not be sufficient to return the movable contact 180 to the “contacts open” position. Thus the electromagnetic switch will be held in the “contacts closed” condition, supplying current continuously to the starter motor.
An additional danger is as follows. When current flow through the coil 110 is halted, the movable contact 180 may adhere to one or both of the contacts 170, 171 due to contact welding, and sufficient force must then be applied by the return spring 210 for overcoming such adherence. However at the stage when the first stationary contact 170 and/or second stationary contact 171 has become completely worn, the sizes, positions and shapes of areas of contact between these contacts and the movable contact 180 will have become substantially changed from original conditions of these. As a result of these changes, if contact welding occurs, the amount of force required to separate the movable contact 180 from the stationary contacts 170 and 171 may exceed the restoring force applied by the spring 210, so that the movable contact 180 will remain held at the “contacts closed” position.